Recent studies suggest that liver is the main site of synthesis of plasminogen, the circulating zymogen of the fibrinolytic enzyme plasmin. We have confirmed these observations by performing in vitro biosynthesis of plasminogen in a cell-free system directed by mRNAs isolated from monkey liver. In order to gain insight into the intraheptic events that occur during the synthesis of plasminogen, we will use a reconstituted heterologous system to mimic the process by which plasminogen is synthesized and vectorially discharged in vivo. Immunochemical techniques will be used for the isolation of the in vitro synthesized precursor forms of plasminogen. These plasminogen forms will be characterized by their binding capacity to L-lysine-Sepharose, reaction with streptokinase and by analysis of their primary amino-acid sequences. We will also study glycosylation, in the presence of endoplasmic reticulum membranes, and analyze the implications of this process in the appearance in plasma of several plasminogen subforms. Antibodies specific for plasminogen will be used for the isolation of polysomal complexes present in the liver, and their mRNAs purified by affinity chromatography procedures.